NRC: emissions choices today have implications for global climate on the scale of millennia

Climate Stabilization Targets: Emissions, Concentrations, and Impacts Over Decades to Millennia, a new report released by the National Research Council (the operational arm of the National Academy of Sciences) on July 16, starkly highlights the long-term global consequences of present-day choices about anthropogenic carbon emissions. The report concludes that “the world is entering a new geologic epoch, sometimes called the Anthropocene, in which human activities will largely control the evolution of Earth’s environment.”

“Choices made now about carbon dioxide emissions reductions will affect climate change impacts experienced not just over the next few decades but also in coming centuries and millennia…Because CO2 in the atmosphere is long lived, it can effectively lock the Earth and future generations into a range of impacts, some of which could become very severe.”

A central conclusion of the report is that it is more effective to assess climate stabilization goals using global mean temperature change as a primary metric, rather than atmospheric concentration levels. One important reason is that evidence suggests that many key climate impacts can be quantified for a given amount of global warming.

As author committee chair Susan Solomon explained in a webinar on the report: “[Climate change] impacts don’t care how much CO2 is in the atmosphere or what the time frame is; they care about what the temperature is.” These projected impacts, calculated per degree Celsius of global warming, include:

• 5-10% changes in precipitation in a number of regions
• 3-10% increases in heavy rainfall
• 5-15% yield reductions of a number of crops
• 5-10% changes in stream-flow in many river basins worldwide
• About 15% and 25% decreases in the extent of annually averaged and September Arctic sea ice, respectively

The report explains:

“Because anthropogenic emissions exceed removal rates through natural carbon sinks, keeping emission rates the same will not lead to stabilization of carbon dioxide. Emission reductions larger than about 80% relative to whatever peak global emissions rate may be reached are required to approximately stabilize carbon dioxide concentrations for a century or so at any chosen target level.”

Solomon also explained that due to lags inherent in the Earth’s climate, climate changes observed as greenhouse gas emissions increase (“transient climate change”) reflect only about half of the eventual total warming (“equilibrium climate change”) that would occur for stabilization at the same concentrations. In other words, our emissions decisions today have the capacity to prevent or lock in warming for centuries or millennia to come.

As is customary for the NRC, the report doesn’t recommend particular emissions reduction targets, “noting that choosing among different targets is a policy choice rather than strictly a scientific one because of questions of values regarding how much risk or damage to people or to nature might be considered too much.”

In addition to it’s focus on temperature increase, the report also suggests the use of cumulative carbon emissions over time as a valuable metric for linking emissions to impacts. The report says:

“The higher the total, or cumulative, carbon emitted and the resulting atmospheric concentration, the higher the peak warming that will be experienced and the longer the duration of that warming. Duration is critical; longer warming periods allow more time for key, but slow, components of the Earth system to act as amplifiers of impacts, for example, warming of the deep ocean that releases carbon stored in deep-sea sediments.”

On the use of a carbon emissions budget as a tool for connecting climate science to policymaking, see our May 25 post, “NRC: US should act now to cut emissions, develop a national strategy to adapt to inevitable impacts.” We reported on the release at the National Academy of Sciences of a set of three new National Research Council reports, part of the larger America’s Climate Choices study requested by Congress. The three reports examine the state of the science of climate change, the scientific underpinnings of domestic mitigation strategies, and approaches to climate change impacts adaptation.

The chief recommendation of the America’s Climate Choices mitigation report, Limiting the Magnitude of Future Climate Change, is that the US use an emissions budget—a specified amount of greenhouse gases that can be emitted over a fixed period of time—as a framework for developing domestic mitigation strategies. This would equip policymakers with a scientific underpinning for developing an emissions reduction regime, without dictating specific policies. This approach explicitly ties scientific understanding of the climate system to policy imperatives, while acknowledging that policymakers need flexibility to design politically feasible emissions reduction strategies.